The state of the art of measuring the thickness of nonmagnetic coatings on a magnetic base using a magnetic field may be ascertained by reference to U.S. Pat. Nos. 2,469,476; 2,625,585; 2,903,645; 3,521,160 and 3,699,487, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 2,469,476 of Sellers discloses an instrument for measuring the thickness of nonmagnetic coatings on a ferrous base and the patent also discloses that it is possible to use the instrument for measuring the irregularities in the surface of a machined ferrous material. According to this patent an adjustable bar magnet bridge is pivoted between the legs of a U-shaped magnet. The limbs of the U-shaped magnet are placed on the surface to be measured. The pivoted bar magnet has an indicator attached thereto and is spring biased so that changes in the magnetic flux of the U-shaped magnet indicate the layer thickness or roughness.
The magnetic measuring gauge disclosed in U.S. Pat. No. 2,625,585 makes use of the principle that forces necessary to disengage a magnet from a workpiece vary in inverse proportion to the thickness of the nonmagnetic plating or coating disposed upon the magnetic base surface. U.S. Pat. No. 2,903,645 makes use of this same principle but in a more complex instrument.
U.S. Pat. No. 3,521,160 is representative of a series of patents of Nix et al which show magnetic thickness gauges using a magnet mounted on the end of a balanced rotatable arm to which a dial is coupled over a spring. The dial is turned until the increasing tension of the spring breaks the magnetic contact and the thickness is read directly from the dial.
Steingroever discloses in U.S. Pat. No. 3,699,487 magnets of anisotropic materials for use in the magnetic thickness gauges.
According to these magnetic thickness gauges of the prior art, the magnet is placed on a coated steel surface and pulled off by a spring. The force required to pull the spring off of the surface is indicated on a scale and expressed in coating thickness. The spring force and the magnetic force are of an uncomplimentary or non-linear nature and produce a logarithmic scale. As time goes on, the magnet either loses or gains magnetism and the spring is subject to fatigue and temperature variations. To obtain a fair degree of accuracy, each prior art gauge has to be individually calibrated and requires an elaborate set-up which is very time consuming. These so-called "pull-off gauges" are calibrated to a mild steel plate. As soon as the coated steel surface is of a rough nature or the substrate is of a different steel than that to which the gauge is calibrated, the readings of these gauges become relative.